The present invention relates to a new thermostable xanthine oxidase and a method of its production. The xanthine oxidase of the present invention is an enzyme which catalyzes the reaction in which hypoxanthine and xanthine are oxidized into hydrogen peroxide, and it is characterized by excellent thermal stability.
The xanthine oxidase of the present invention is used to quantitatively determine inorganic phosphorus which serves as an index of, for example, nephritis and thyropathy, adenosine deaminase which serves as an index of immune diseases, and other substances.
Heretofore, xanthine oxidase has been known to be present in cow's milk, but quantitative determination of the xanthine oxidase derived from cow's milk on the basis of hydrogen peroxide wherein the xanthine oxidase is allowed to act on a sample containing hypoxanthine and xanthine and the resulting hydrogen peroxide is converted to a pigment has been unattainable because the pigment is decomposed in a short time.
In addition, microbial xanthine oxidase enzymes have been known, including those derived from microbes belonging to Pseudomonas, Escherichia, Arthrobacter, Nocardia and other genera [J. Bacteroiology, 130, 1175 (1977); J. Bacteriol., 135, 422 (1978)] and the xanthine oxidase enzyme derived from Enterobacter cloacae [A.B.C., 45, 425 (1981)], but none of them have sufficient activities or thermal stability.